Data transmission between a transponder, such as a smart card, and a read/write station already has existing standards, such as ISO 14443, some of which are still being worked on. These standards regulate, among other things, the modulation type for the RF signals transmitted between the card and the read/write station, the carrier frequency for said RF signals and the coding of the transmitted data.
The standard ISO 14443 provides for data transmission to involve the use of RF signals onto which the data to be transmitted are modulated on the basis of ASK modulation (ASK=amplitude shift keying). In this context, the RF signal alternates between an upper and a lower amplitude value on the basis of the data signal to be transmitted. Two modulation methods are provided whose signals respectively have differing degrees of modulation. In this context, the degree of modulation is calculated from the quotient between the difference and the sum of the upper and lower amplitude values. In the case of “ASK100% modulation”, which is also called hard keying or on-off keying, the lower amplitude is zero, the degree of modulation is thus 100%, and the RF signal is thus fully turned off on the basis of the data signal. In another method, which has the working title ASK10%, the degree of modulation is between 5% and 15%, and the lower amplitude value is thus between approximately 74% and 90% of the upper amplitude value. As compared with ASK100% modulation, ASK10% modulation has the advantage that the spectrum of the signals to be transmitted has a narrower bandwidth, but it has the drawback of greater susceptibility to interference, and hence of a shorter possible transmission distance between the read/write station and the transponder.
To process the modulated energy signal, known smart cards have the following components: a receiver for receiving the modulated RF signal, a signal processing unit connected downstream of the receiver for obtaining the data signal from the RF signal using demodulation and decoding, and a data processing unit to which the data signal can be supplied.
Besides transmission of the modulated data signal, the RF signal is used for supplying voltage to the smart card. To this end, the RF signal is rectified in the signal processing unit and is supplied to a voltage regulating arrangement which corrects fluctuations in the received power of the RF signal in order to make available a supply voltage which is as constant as possible.
A drawback is that voltage regulators for producing a supply voltage from an ASK10% modulated signal and voltage regulators for producing a supply voltage from an ASK100% modulated signal are not compatible. In particular, a regulator for ASK10% is not suitable for processing signals from ASK100% modulation. With this modulation type, signal pauses arise, whereas ASK10% modulation involves a reduction in the signal amplitude to a minimum of 74%. The ASK10% regulator is not designed for such signal pauses and is not capable of reliably maintaining the supply voltage on receiving ASK100% modulated RF signals.
Hence, the only methods for contactless data transmission which are known to date are those in which the transponder is capable of processing RF signals modulated on the basis of one of the modulation types ASK100% or ASK10%. If the transponder is provided with circuit measures for differently modulated and/or coded RF signals, the particular problem which arises is that of identifying the modulation and/or coding type for the received RF signals and of setting the transponder to process the identified signal.